cmd_vel or commands/velocity?

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I'm trying to use the existing amcl/gmapping and create my own wheel motor drive actuator. it seems that the output from navigation (amcl/gmapping) are fed into move_base http://wiki.ros.org/move_base

the above wiki shows that move_base dumps the velocity commands to /cmd_vel

but on the turtlebot simulator, I see the commands finally go to */commands/velocity, in fact the /cmd_vel topic doesn't exist at all.

so which one is it really? I just need to know the correct place for my actuator to get its commands from.

also gmapping does SLAM so supposedly it can also do localization, what is the reason we use a separate navigation package (amcl) ?

thanks Yang